Recently, with the development of various mobile devices and IT devices, in addition to a keyboard and a mouse, various types of easy-to-use and efficient input devices have been used for easy-to-use and efficient input. In particular, the most commonly used input device is a touch screen panel.
Such a touch screen panel is provided on a display surface of an image display device such as a smart phone, an electronic notebook, a computer, or a terminal so that a user can easily input or select desired information while viewing an image.
The above-described touch screen panel requires a transparent electrode in a form of a surface electrode having overall transmittance and electrical conductivity in order to sense user's touch. In the related art, techniques of depositing indium tin oxide (ITO) that is an inorganic material as a transparent electrode have been mainly used. Although the ITO has excellent electrical conductivity, indium is a rare earth metal, and its price has sharply risen. Furthermore, since the ITO is manufactured through a deposition process, there is a problem in that the ITO has limitations in mass production and large area. Due to these problems, research and development of new materials substituted for the ITO and new processes has been urgently required.
As new materials substituted for the ITO, there have been proposed conductive inks in which metal nanoparticles are easily dispersed, metal nanowires, graphene, carbon nanotubes (CNT), conductive polymers, and the like. Among methods of using these materials, a method of forming a transparent electrode in which metal wires having a thin line width are formed in a metal mesh structure by using a conductive ink based on metal nanoparticles, a method of forming a transparent electrode by applying metal nanowire dispersion solution have drawn much attention as techniques for ITO substitution due to excellence in electric conductivity and simplicity of process.
However, an electrode made of metal has a problem that the electrode may be visually recognized by a user due to the opacity of metal and is difficult to be used as a transparent electrode. In order to solve such a problem and to prevent the electrode from being recognized by the user, a metal wire having a line width of micrometers which is difficult for the user to visually recognize may be formed in a mesh structure.
In this case, in order to improve transmittance as a main parameter of the transparent electrode formed with a mesh structure of the conductive wire and to secure excellent visibility to a image on a display so that the user cannot visually recognize the electrode, the line width of the conductive wire to be printed is required to be as fine as 3 μm or less.
As a typical process of forming a printing-based metal mesh satisfying the above-described condition of the line width of the wire, there are gravure printing and offset printing. These processes have an advantage in that the processes are excellent in mass productivity. However, as the line width of the conductive wire having a mesh structure is formed to be fine in order to improve the transmittance, the sheet resistance is increased and, thus, the conductivity is decreased. In other words, there is a trade-off relationship between the transmittance and the conductivity.
In order to solve the above-mentioned trade-off relationship between the transmittance and the conductivity, while allowing the line width of the conductive wire to be fine, the thickness of the conductive wire is required to be increased. However, in the gravure printing and offset printing in the related art, the thickness of the conductive wire may be limited to be in a range of several tens of nanometers to several hundreds of nanometers due to the viscosity and process limitations of the conductive solution to be printed.
Therefore, the printing process in the related art has a problem in that the process has limitations in improving the sheet resistance characteristic and realizing excellent conductivity in the formation of the fine conductive wire for improving the transmittance.
FIG. 1 is a cross-sectional diagram conceptually illustrating that natural light is reflected on a front surface, a side surface, or the like of a metal wire in a transparent electrode in which a metal wire is formed in a mesh structure in the related art. Since the transparent electrode element configured with the metal wire having a mesh based on the metal nanoparticles or the transparent electrode element based on the metal nanowires described above are made of a metal material having good reflectance with respect to light, as illustrated in FIG. 1, natural light is reflected on the front surface and the side surface of the transparent electrode elements. As a result, there is a problem in that the visibility is lowered.